Measurement while drilling (MWD) allows for the surface acquisition of downhole data during drilling, thereby reducing the need for costly and time consuming drill string tripping and logging/survey runs otherwise necessary to acquire downhole data.
MWD systems typically include the placement of a complex and expensive self-contained package of sensors, power supplies and transmitters very near the drill bit. While this position is desirable, it presents a harsh, hot, highly pressured, dirty and high shock load environment for the MW) tool. MWD tool failures are not uncommon, requiring retrieval and replacement of the tool downhole. In the event the drill pipe becomes stuck in the hole, the MWD tool may be permanently lost. As downhole vibrations act on retrievable MWD tools, the modules of the tool have a tendency to rattle or bang against the internal walls of the surrounding MWD receptacle thereby amplifying the vibrations. Thus, to reduce the potential damage caused by these downhole shocks, it is desirable to centralize and secure the retrievable MWD systems within their surrounding MWD receptacles.
One common method of improving the centralization of MWD tools involves the use of donut-shaped rubber rings. These rubber rings are placed around the circumference of MWD tools to increase their outer diameter and cushion some of the shock effects described above. However, the rings must be sized to pass through the smallest drill string restriction; otherwise, a tool could not pass through the restriction. This sizing limitation results in a less than tight fit between the tool and the surrounding receptacle, which hinders the effectiveness of the rings.
Another device for centralizing MWD tools is described in U.S. Pat. No. 5,348,091. That device utilizes a single upper centralizer having a housing which is attached to a MWD tool while at the surface. The combined apparatus is lowered through the drill string by wireline releasably attached to a sliding shaft. During lowering, the weight of the housing and tool extends the slidable shaft, positioning extendable fingers above a tapering nose on the housing in a retracted position. When the tool lodges downhole, the slidable shaft moves downward, causing the fingers to move outward against the drill pipe. During drilling operations, mud flowing through the drill pipe acts downwardly on a flared portion of the slidable shaft, thereby inducing more forcible extension of the fingers against the drill pipe and maintaining centralization of the tool. Retrieval of the tool is typically made by interrupting mud flow and lifting the centralizer through the drill string by wireline. Although this device has been more successful than the rubber rings, an improved middle centralizer is desirable for fairly long tools including tools other than MWD that are lowered through conduit and retrieved on wireline.